The following discussion of the background to the invention is intended to facilitate an understanding of the invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was published, known or part of the common general knowledge as at the priority date of the application.
Metal-Organic Frameworks (MOFs) are a class of promising porous materials having tuneable functionality, large pore sizes and the highest known surface areas. These characteristics are of high interest for a myriad of industrial applications such as gas storage, gas separation, drug delivery and catalysis. However, to date the cost of these materials has remained prohibitively high, thereby restricting the ability of these materials to make a significant impact on prospective markets or technologies. Very few MOFs described in academic literature are commercially available, with that availability limited to small quantities (grams).
An important requirement for accessing the potential applications of MOFs is the ability to routinely synthesise MOF materials in large quantities (kg scale or higher) at an economic price point. Such a process needs to be a versatile, efficient and scalable synthesis that is able to produce MOFs in large quantities in order to introduce these materials to real world applications.
However, traditional laboratory routes such as the classical solvothermal synthesis are difficult to scale up due to the extended reaction times (˜24 hours) and low quality material yield. Furthermore, a wide variety of available synthetic synthesis methods have a singular nature providing an inherent inflexibility for any prospective production process.
One of the barriers to scaled-up MOF synthesis is that commonly MOFs nucleate at a reaction surface, meaning that the size of the reaction vessel becomes a significant parameter in the synthesis conditions. Consequently, reactions that proceed in small lab scale conditions are not always successful when scaled up into larger vessels, limiting scaled up MOF chemistry to a small number of MOFs that are robust in their preparation, each requiring bespoke equipment. Therefore a method to conveniently expand the scale of production, keep sufficient residence times, while minimising vessel geometry is extremely attractive to applied MOF chemistry, offering a versatile route to production.
Continuous flow chemistry is renowned as a paradigm shifting approach to chemical synthesis. The improved heat and mass transfer available often leads to improved reaction yields, reduced reaction times, faster reaction syntheses, new synthetic pathways, and broader green chemistry implications.
Recent studies have reported that it is possible for MOFs to be produced by continuous processes. Gimeno-Fabra M. et al. Instant MOFs: continuous synthesis of metal-organic frameworks by rapid solvent mixing. Chem. Commun. 48, 10642-10644 (2012) showed that use of a bespoke tube-in-tube, counter-current mixing reactor at the high temperature of 300° C. can lead to MOFs. It was also shown that small amounts of MOFs, within oil droplets, can be made in microfluidic reactors (see Faustini M. et al. Microfluidic Approach toward Continuous and Ultra-Fast Syn-thesis of Metal-Organic Framework Crystals and Hetero-Structures in Confined Microdroplets. J. Am. Chem. Soc. 135, 14619-14626 (2013) and Paseta L. et al. Accelerating the controlled synthesis of MOFs by a microfluidic approach: a nanoliter continuous reactor. ACS Appl. Mater. Interfaces 5, 9405-9410 (2013)). In 2013 Kim K.-J. et al. (High-rate synthesis of Cu-BTC metal-organic frameworks. Chem. Commun. 49, 11518-11520 (2013)) reported a proof of concept mesoscale flow production of HKUST-1 using a continuous flow reactor comprising a 30 cm long and 1.59 mm I.D. stainless steel pipe. It is noted that the MOFs produced had moderate surface area at low scale. All of these early reports are promising steps towards production of MOFs at scale. However, in order for this to be viable, pure MOFs must be readily attainable without a loss in product quality.
Given the wide array of MOFs known, and the likelihood of a large range of applications each requiring different MOFs in the future, a versatile production technique is crucial. It would therefore be desirable to provide a new and/or improved method and apparatus for producing MOFs.